Depositing a tantalum film

ABSTRACT

This invention relates to a method of depositing a tantalum film in which α-Ta dominates and to methods of electroplating copper using such films. The films have a thickness of less than 300 nm and are formed by depositing a seed layer of an organic containing low dielectric constant insulating layer and sputtering tantalum onto the seed layer at a temperature below 250° C.

This, invention relates to a method of depositing Ta film in which α-Tadominates and methods of electroplating copper utilising tantalum films.

Sputter deposition of Tantalum thin films onto insulators in an inertgas atmosphere generally leads to the formation of β-Ta. This phase,which possesses a tetragonal structure, is the high-resistivity Taphase, with a value in the range of 180–220 μΩcm.

A second phase is α-Ta, with a bcc structure and a much lowerresistivity of approximately 20–50 μΩcm. Thirdly, a mixture of bothphases can be found in Ta thin films with resistivity values betweenthose of the pure phases.

The low resistivity of α-Ta makes it the favourable candidate over β-Tafor numerous applications in the electronics industry. Although α-Ta isthe thermodynamic stable phase in the bulk metal, it is very difficultto reproducibly generate the low-resistivity phase in sputtered thinfilms with a typical thickness of up to 300 nm. Here, the highresistivity β-phase dominates.

There are three methodologies known to the prior art that may beemployed to produce pure α-Ta thin films by sputtering.

Firstly, a substrate temperature exceeding 600° C. can lead to theformation of α-Ta. This approach however is of little practical use forelectronics applications, where substrate temperatures often have to beminimised to avoid damage to temperature sensitive materials.

The second method involves the introduction of gaseousimpurities/foreign atoms into the Ta lattice during the sputterprocesses or in an additional post-deposition plasma treatment. Theseimpurities include nitrogen, hydrogen or oxygen. However, the change inresistivity with the amount of gaseous impurities can be very rapid andcontrol is generally difficult.

The third and most recent approach is the introduction of an additionalbase layer, which acts as a ‘seed’ for the formation of thelow-resistivity α-phase in the subsequently deposited Ta film. Thesebase layers include TaN, Nb, or W.

These methods are either impractical or involve the, introduction of anadditional process step/material. Furthermore, the addition of a secondlayer in the last case might considerably increase the resistivity ofthe complete layer stack (base layer and Ta).

Tantalum is of particular interest in a copper metallisation in asubmicron damascene structure for microelectronic applications.Typically there is a barrier layer consisting of firstly tantalum andthen tantalum nitride, followed by a seed layer of copper. This is thenplated to fill the damascene structure with copper. The tantalum isprimarily an adhesion layer. There is therefore a need for a lowtemperature method for forming alpha phase tantalum.

WO-A-00/17414 describes depositing low resistivity tantalum and tantalumnitride layers at an elevated substrate temperature of about 325° C. toabout 550° C. In particular on page 14 of that disclosure a tantalumfilm of low resistivity was deposited at a substrate platen temperatureof 400° C. that is said to equate to a substrate temperature of 325° C.to 350° C. or higher. Tantalum deposited at lower temperatures is shownto be of high resistivity and to have been converted by a subsequentanneal of about 400° C. or higher to a low resistivity film. Thereforethis disclosure shows the creation of low resistivity tantalum only whenthe wafer is subject to the application of 400° C.

Whilst it is stated that low resistivity tantalum is alpha phase,nowhere does this disclosure describe what the surface of the substrateconsists of upon which the tantalum and tantalum nitride is formed, northe thickness of the tantalum films deposited. It is already well knownthat the receiving surface and film thickness are importantconsiderations in the formation of alpha tantalum films.

We will also show later that higher resistivity tantalum does notnecessarily mean the loss of the alpha phase of tantalum as is impliedin this disclosure.

Turning to the question of temperature, as the relevant criteria isthermal budget, there is little practical advantage in depositing atroom temperature and then subsequently annealing at a highertemperature, over depositing at that higher temperature in the firstplace.

The present invention relates to a method of depositing Ta film in whichα-Ta dominates having a thickness <300 nm and more particularly lessthan 30 nm (300 Å) comprising depositing a seed layer of an organiccontaining low dielectric constant insulating layer and sputteringtantalum onto the seed layer at a temperature below 600° C.

Preferably the temperature is below 250° C. and a temperature ceiling of100° C. is particularly preferred.

Preferably the seed layer is a carbon doped hydrogenated silicon dioxidee.g. methyl-doped silicon oxide and particularly conveniently it may bea film deposited in accordance with the teaching of WO 01/01472, whichis in the name of the same applicants, the teaching of which is herebyincorporated. Thus the seed layer may be formed by reacting asilicon-containing organic compound or compounds and oxidising agent(s)in the presence of a plasma and setting the resultant films such thatthe carbon-containing groups are contained therein. The film may be setby exposing it to a hydrogen-containing plasma.

In a particularly preferred embodiment, the surface of the seed layerhas been etched away prior to sputtering. Additionally or alternatively,the sputtered material may arrive at the surface of the seed layer withhigher energy e.g. by the application of substrate bias and/or ionisedsputtering. Long throw sputtering with a target to substrate spacing ofat least 200 mm and preferably >240 mm is preferred. The materialdescribed as the seed layer is in fact a very effective low k dielectricmaterial and therefore may already exist in, for example, asemi-conductor device during its manufacture. In that case the layerwould function both as the low k layer and the effective seed layer forthe Ta, particularly after a surface crust layer is removed e.g. of10–50 nm e.g. by a plasma etch, of C₃F₈ for 30 seconds. Such surfacelayers may not be representative of the bulk seed layer eitherstructurally and/or chemically.

It has been found by the Applicants that these films are particularlysuitable for using on profiled surfaces such as the profiled surface ofa semiconductor wafer during the manufacture of semiconductor chips, forexample using the dual damascene process.

The trend is to use copper in such processes and to deposit it byelectroplating. To achieve a good and uniform plating process it ispreferable that the conductive layer on which the copper is to beelectroplated, has a low and preferable uniform electrical resistance.This is because electrical contact with the front side of the wafer ismade via clips at the edge of the wafer. As wafer sizes are now up to300 mm and the front side of the wafer is profiled with recesses, theelectrical resistance across the wafer would otherwise be variable andsufficiently large as to cause plated thickness non-uniformity.

Typical tantalum barriers are of high resistivity as they are generallyof beta phase tantalum. This phase has a resistivity of typically around170˜200 micro.ohm.cm. Accordingly, to achieve good electroplating,normal practice is to sputter a copper seed layer of low resistivity,before electroplating copper to fill the recesses. This extra stepobviously decreases throughput and adds to cost, because of the need toprovide additional chambers or even additional sputtering systems forthe copper seed layer

Thus, from a further aspect, the applicants' invention consists in amethod of electroplating copper onto a profiled surface of asemiconductor wafer or the like, including depositing a tantalum layer(typically required as a copper diffusion layer) onto the profiledsurface and electroplating a copper layer directly onto the tantalumlayer, wherein the tantalum layer has a resistivity of less than 50micro.ohm.cm.

Preferably the tantalum barrier layer is at least substantially alphaphase tantalum.

The resistivity of the thin film layer (typically under 3000 Å thick butusually more than 50 Å thick) may be between 20 and 40 micro.ohm.cm andmost particularly is about 25 micro.ohm.cm though the lower theresistivity the better. Bulk α phase tantalum resistivity is 13micro.ohm.cm.

It will be appreciated that the tantalum layer can be deposited usingthe methods described above or in the following description.

Although the invention has been defined above, it is to be understood itincludes any inventive combination of the features set out above or inthe following description.

The invention may be performed in various ways and specific embodimentswill now be described, by way of example, with reference to theaccompanying drawings in which:

FIG. 1 is a summary of experimental detail;

FIGS. 2 a, b and c are respective x-ray defraction (XRD) patterns forcertain of the films resulting from the described experiments;

FIG. 3 is a Secondary Ion Mass Spectroscopy (SIMS) plot for a TMS+O₂deposited film which has been set with a five minute hydrogen plasma;

FIG. 4 is a schematic view of an AHF deposition chamber configuration;

FIG. 5 is a graph of dielectric thickness against the resistivity of thetantalum layers showing the effect of an inert sputter etch;

FIG. 6 corresponds to FIG. 5 for cases where the dielectric layer hasbeen reactively etched back; and

FIG. 7 shows the surface roughness resultant from the etching of FIG. 6.

As has already been mentioned WO 01/01472 describes a method ofdepositing a low k insulating film.

As can be seen from FIG. 3, which is a SIMS diagram of such a film thereis a top part, seen at between approximately 0–800 Å, which ischemically variable and distinct from the bulk of the film. In theexperiments described below samples were prepared by depositing thisinsulating material onto respective substrates and then some of thesamples had approximately 30 nm of this upper layer reactively plasmaetched away using a C₃F₈ etch. For comparison purposes further sampleswere prepared on which a layer of thermal silicon dioxide was depositedinstead of the low k film.

Ta was then deposited onto the samples under various process conditionsand the resultant Ta film was analysed by x-ray defraction (XRD) todetermine which phase of Ta had been deposited.

The experimental set up was varied in a number of ways which areidentified in FIG. 1, in accordance with the following key:

AHF: A source to substrate distance of 245 mm, tantalum target with apower of 5–25 kw d.c. applied, argon sputter gas at a pressure of 5millitorr or less. Significant target material ionisation occurs in thisprocess whereby a magnetically confined moving magnetron is used. Thisarrangement is described more fully in connection with FIG. 4.

HF: the same process conditions but without significant target materialionisation. Experiments have been carried out (not shown) that indicatethat these longer source to substrate distances (standard for magnetronsputters is about 45 mm ) improve the results.

Bias: 13.56 meghz applied to the wafer platen to induce a dc voltage.

ULK: The film of WO 01/01472.

Crust: The uppermost 500 Å (=50 nm) of the low k film is present andforms the substrate surface upon which the tantalum has been magnetronsputtered.

No Crust: The uppermost surface has been reactively plasma etch removedwith e.g. C₃F₈ or CF₄.

It will be noted that prior to sputtering, in a number of cases, thewafer was heated to 200° C. to encourage outgassing.

The chart of FIG. 1 summarise experiments performed using the followingconditions:

AHF+Bias

-   -   13 kw DC target power    -   100 sccm of Argon    -   300 watts 13.56 Meg RF bias power to platen including a voltage        of ˜75v negative    -   Target magnetron confinement magnetic coil powered at 1,360        ampere turns    -   Process time 46 seconds for 150 nm deposition    -   Platen coils were powered at 320 ampere turns to improve        uniformity.

HF

-   -   13 kw DC target power    -   100 sccm of Argon    -   No power applied to the substrate platen and no magnetic        confinement of the target magnetron    -   Process time of 29 seconds for 140 nm deposition

HF & Bias

-   -   As HF with 100 watts of 13.56 meg RF bias power to the platen        inducing a voltage of ˜78v negative    -   Process time of 33 seconds for 150 nm deposition.

It will be noted, from FIG. 1, that under all conditions, when the crustof the ULK film was reactively etched away, the alpha phase predominatedeither in combination with the beta phase or to the exclusion of thatphase. In all cases a significantly lower resistivity was achieved. Itis postulated that this surprising selectivity of deposition is a resultof the chemical make up of the underlying layer, which can be seen bothfrom FIG. 4 and from the chemical composition given by Rutherford BackScattering (RBS) as can be seen in the following table: Chemicalcomposition by Rutherford Back Scattering (RBS)

Hydrogen Carbon Oxygen Silicon Depth percent percent percent percent<800Å 32 17 33 18  800–1700Å 42 16 25 17 1700–3500Å 46 14 25 15

Thus where at least part of the crust is etched away, there issignificantly more hydrogen present at the surface and/or it is moreable to diffuse to the surface because of the at least partial removalof a dense upper layer and it is postulated that the hydrogen may bedesorbed from the substrate and incorporated into the growing Ta filmduring the initial stages of film growth. This, it is conceived maycause the preferential deposition of the alpha phase. This could furtherexplain why, when the insulating layer is heated preferential depositionof α-Ta takes place, because if the process is thermally activated, thenthe heat may compensate for the lower level of hydrogen in the crust.

By carefully adjusting the deposition parameters a mixture of bothphases can be formed with an intermediate resistivity value, as shown inthe chart.

It should also be noted from FIG. 1 that at a deposition temperature of200° C. when the AHF+Bias process was run then an alpha phase tantalumof low resistivity was deposited upon the crust.

An important difference between the AHF and HF processes is the energylevel at the tantalum film during deposition. The unbalancing effect onthe magnetron by the use of the coils 10 increases plasma ion flux (bothsputter gas and metal) from an ion density of about 5 mAcm² to about 20mAcm² and allows a significant part of the plasma to reach the substratesurface. This increasing energy level effectively substitutes for andavoids the need for the higher substrate temperatures of earlierdisclosures. 200° C. is however considered to be too high a preferabletemperature for a barrier deposition process and the focus of laterexperiments shown here was on processes of less than 100° C. that couldreliably yield alpha phase tantalum.

FIG. 2 a shows the make up of a 100 nm thick Ta film deposited on thelow-k insulating layer made in accordance with WO 01/01472 and it willbe seen that there is significant deposition of α-Ta to the exclusion ofβ-Ta. FIG. 2 b shows the corresponding results where the Ta film wasdeposited on the thermal silicon dioxide. FIG. 2 c indicates the resultswhere the deposition parameters have been adjusted so that both phasesare deposited on the low-k dielectric.

It will also be noted from the table in FIG. 1 that the higher energypresent in the growing sputtered film of the AHF apparatus appears to bemore successful than the non ionised arrangement designated at “HF”.This suggests that the high energy within a growing sputtered film canliberate a seeding agent from a seed layer without the use of hightemperatures.

It is believed that, quite apart from the reduced resistivity, there isa further advantage in using α-Ta in that it promotes a (111) texture ina subsequently deposited copper seed layer. This is the preferredorientation for copper seed layers as it promotes (111) texture in theelectroplated copper and this is believed to be more resistant toelectromigration. Comparing intensities of (111) texture expressed as aratio of peak intensity {111} over peak intensity {200} gives thefollowing results:

$\text{Copper~~seed~~deposited~~onto~~alpha~~tantalum} = {\frac{I\left\{ 111 \right\}}{I\left\{ 200 \right\}} = 368}$$\text{Copper~~seed~~deposited~~onto~~beta~~tantalum} = {\frac{I\left\{ 111 \right\}}{I\left\{ 200 \right\}} = 61}$

The AHF configuration is more particularly described in ourInternational Application No. PCT/GB01/03229, the contents of which areincorporated herein by reference. For convenience a copy of FIG. 1 ofthat application is incorporated herein as FIG. 4. Briefly a movingmagnetron 1 is provided above a target 2, which faces a platen 12 onwhich is mounted a substrate 3. An external coil 10 is provided in thevicinity of the periphery of the target 2 to alter the normal magneticfield applied by the moving magnetron 1. This coil 10 can be used tounbalance the magnetron and hence allow direct control of the ion-fluxreaching the growing film, whereas a separate RF wafer-bias system 11 isused to control the energy of the impinging ions.

In the HF arrangement the coil 10 is either not present or is notenergised.

Whilst these are the two sputtering arrangements so far experimentedwith they do not limit the generality of the invention. A single wafersputtering system is particularly preferred whereby the wafer is notlaterally moved during deposition. Any arrangement whereby there is ahigh level of energy provided to the tantalum film and/or the surfaceupon which it grows is included, where this energy does not take thebulk wafer temperature above about 250° C. and most preferably not aboveabout 100° C.

Further analysis of the work reported in the table of FIG. 1 suggeststhat the fact that the AHF configuration was operated in an unbalancedmagnetron mode, whilst the HF configuration was operated in a balancemode may be influential on the type of film deposited. Additionally oralternatively, the fact that the AHF is a long throw arrangement mayalso influence the type of film deposited. What is desirable is thatthere should be a high level of activation at the deposition surface.

Subsequent experiments described in FIGS. 5 and 6 have been carried outbroadly in accordance with the earlier AHF experiments at a source tosubstrate distance of 245 mm and a wafer platen temperature of 75° C.

These experiments were designed to investigate the well-known ‘thinfilm’ effect (particularly important for thin film barriers) and theeffect of chemical compositional changes in the surface upon which thetantalum was sputtered. The low-k layer is known to have changes incomposition as shown in FIG. 3 and additionally as the layer is etchedback (as shown in FIG. 6) the resultant surface roughness increases asis shown in FIG. 7.

All the experiments shown in FIGS. 5 and 6 were carried out with adielectric material as output from that deposition system. This systemgenerally removes a surface layer by means of a C₃F₈ plasma etch for 30seconds thereby removing at least some, but not necessarily all of anupper layer of the dielectric that from FIG. 3 is known to be of adifferent chemical composition. In FIG. 5 therefore the results labelled“Before Argon Etch, as-deposited dielectric” refer to the dielectriclayer with at least some of the surface crust layer removed.

In FIG. 5, in addition to the ‘thin film’ effect can be seen thesurprising result that an inert argon etch of the low-k dielectric bothincreases tantalum resistivity and shifts it from being always alpha tomixed alpha and beta. It is not yet known why this is happening, but isprobably caused by a surface effect such as roughness or selectiveremoval (by sputter etch) of components of the dielectric layer. Notehowever that the resistivity of the tantalum does not greatly change asthe dielectric layer is etched.

By contrast reactive plasma etching of the low-k dielectric was carriedout with CF₄+CH₂F₂, the results of which are shown at FIG. 6. It shouldbe noted, by reference to FIG. 3, that this is an etching through aregion of varying chemical compositions into the ‘bulk’ material and byreference to FIG. 7 is increasing surface roughness of the surface uponwhich the tantalum is to be deposited. In FIG. 6 it can be seen that areactive CF₄+CH₂F₂ etch unlike the argon sputter etch does not promotebeta tantalum. Rather there is always alpha phase tantalum, theresistivity being near bulk for the thickest film here, 68 nm. Alsoshown is that as the dielectric layer is etched into the resistivity ofthe subsequently deposited tantalum increases. This is particularly thecase for the thinner films shown here where the increase in resistivityis of the order of 30%–50%. For 28 nm thicknesses and above the effectis much reduced and somewhat slight for the thickest film of 68 nm. Itis known that the structure of the low-k dielectric layer is nothomogeneous and this would indicate that for (in particular) very thinfilms the structure of the surface layer upon which the tantalum issputtered is important to its morphology.

Whilst it is clear that the surface of the substrate plays a criticalpart in the morphology of the sputtered tantalum film it is not yetknown what (relative) parts are played by structure or chemistry. Thelow-k dielectric film is of the SiCO:H type, that may be considered ahydrogenated carbon containing silicon dioxide. Carbon and/or hydrogenmay be seeding the alpha phase and/or this seeding may be as a result ofthe microstructure of the surface of the dielectric. In addition thereis fluorine present from the etch processes and this may also be playingsome part.

Accordingly, the present invention includes a method of depositing Tafilm in which alph-Ta dominates having a thickness of <300 nm comprisingdepositing a seed layer of a low dielectric constant insulating layerhaving carbon and/or hydrogen present in its near surface region or anappropriate microstructure in that region and physical vapour depositingtantalum onto the surface of the seed layer at a temperature below 250°C.

Preferably the tantalum film is no thicker than 30 nm and the depositiontakes place at or below 100° C.

No subsequent anneal is required in order to achieve the low resistivityassociated with alpha tantalum.

In a further preferred feature the physical vapour deposition process issputtering most preferably a single wafer sputtering system where thereis no lateral movement of the wafer during the deposition process.Preferably the process utilises a sputter chamber having an unbalancedmagnetron. It is particularly preferred that the “unbalancing” isachieved using an electromagnetic coil whereby direct control of the ionflux to the substrate can be achieved, particularly with a view toachieving high actuation at the deposition surface. Other mechanisms forachieving such direct control may be equally applicable.

Whilst an ionised sputter system of an unbalanced magnetron type hasbeen used for these experiments any ionised metal sputter system thatachieves the necessary conditions to achieve the results shown here areincluded. Such systems include ‘ion metal plasma’, ‘hollow cathodemagnetron’ and ionised PVD where in all cases a high degree of sputteredmetal is ionised compared to standard sputtering systems.

For the sake of clarity it should be understood that the seed layer ofthis invention is not necessarily a separate layer. The seedingcharacteristics necessary for this invention may be carried out by alayer that also serves other useful purposes, such as a dielectric,barrier or etch stop layer within a damascene structure.

It should further be understood that references to ‘insulatingsubstrates’ refers to the characteristics relevant to this patent, beingan insulating surface upon which the tantalum is deposited.

1. A method of depositing Ta film in which α-Ta dominates having athickness <300 nm comprising depositing a seed layer of anorganic-containing low dielectric constant insulating layer andsputtering Tantalum to deposit a-Ta onto the seed layer at a temperaturebelow 250° C., wherein the seed layer is a methyl-doped silicon oxide.2. A method of depositing Ta film in which α-Ta dominates having athickness <300 nm comprising depositing a seed layer of anorganic-containing low dielectric constant insulating layer andsputtering Tantalum to deposit a-Ta onto the seed layer at a temperaturebelow 250° C. wherein the seed layer is formed by reacting a siliconcontaining organic compound and an oxidising agent in the presence of aplasma and setting the resultant film such that the carbon containinggroups are contained therein.
 3. A method as claimed in claim 2 whereinthe seed layer is set by exposing it to an hydrogen containing plasma.4. A method of depositing Ta film in which α-Ta dominates having athickness <300 nm comprising depositing a seed layer of anorganic-containing low dielectric constant insulating layer andsputtering Tantalum to deposit a-Ta onto the seed layer at a temperaturebelow 250° C. wherein the surface of the seed layer has been etched awayprior to tantalum deposition.
 5. A method of depositing Ta film in whichα-Ta dominates having a thickness <300 nm comprising depositing a seedlayer of an organic-containing low dielectric constant insulating layerand sputtering Tantalum to deposit a-Ta onto the seed layer at atemperature below 250° C. wherein the seed layer is heated prior totantalum deposition.
 6. A method of depositing Ta film in which α-Tadominates having a thickness <300 nm comprising depositing a seed layerof an organic-containing low dielectric constant insulating layer andsputtering Tantalum to deposit a-Ta onto the seed layer at a temperaturebelow 250° C. wherein the tantalum source to substrate separation is atleast 200 mm.
 7. A method as claimed in claim 6 wherein the tantalumsource to substrate distance is between 200 mm and 250 mm.
 8. A methodof depositing Ta film in which α-Ta dominates having a thickness <300 nmcomprising depositing a seed layer of an organic-containing lowdielectric constant insulating layer and sputtering Tantalum to deposita-Ta onto the seed layer at a temperature below 250° C. wherein thetantalum deposition is by unbalanced magnetron sputtering.
 9. A methodof depositing Ta film in which α-Ta dominates having a thickness <300 nmcomprising depositing a seed layer of an organic-containing lowdielectric constant insulating layer and sputtering Tantalum to deposita-Ta onto the seed layer at a temperature below 250° C. wherein thesubstrate is biased.
 10. A method of depositing Ta film in which α-Tadominates having a thickness <300 nm comprising depositing a seed layerof an organic-containing low dielectric constant insulating layer andsputtering Tantalum to deposit a-Ta onto the seed layer at a temperaturebelow 250° C. wherein copper is deposited on the Ta film.
 11. A methodof depositing Ta film in which α-Ta dominates having a thickness <300 nmcomprising depositing a seed layer of an organic-containing lowdielectric constant insulating layer and sputtering Tantalum to deposita-Ta onto the seed layer at a temperature below 250° C. wherein the seedlayer is an inter metal dielectric layer.
 12. A method of depositing Tafilm in which α-Ta dominates having a thickness <300 nm comprisingdepositing a seed layer of an organic-containing low dielectric constantinsulating layer and sputtering Tantalum to deposit a-Ta onto the seedlayer at a temperature below 250° C. wherein a significant proportion ofthe tantalum material arrives at the seed layer in an ionised state.